1. Field of the Invention
The present invention relates to photoelectric conversion elements with the photovoltaic effect which are suitably applicable to solar cells and photosensors. Solar cells are used as components for solar light power generation systems, electronic calculators, and night indicator lamps, for example. Also, photosensors are employed as components of facsimile apparatuses, image scanners, and factory automation (FA) equipment.
In particular, the present invention relates to photovoltaic elements of the pin-type composed of non-single crystalline silicon type semiconductor material. Also, this invention relates to photovoltaic elements having an amorphous silicon-germanium semiconductor layer in the photovoltaic layer. Also, this invention relates to photovoltaic elements containing microcrystalline germanium. Also, this invention relates to photovoltaic elements wherein the doping layer (p-layer and n-layer) and the i-layer are formed by microwave plasma CVD (MWPCVD).
2. Related Background Art
In recent years, much research has been conducted for the purpose of using amorphous silicon-germanium in photoelectric conversion elements, e.g., photovoltaic elements. For example, a photovoltaic element for efficiently converting long wavelength light into electric power by using an amorphous a-SiGe layer for photoelectric power generation, or a photosensor using an a-SiGe layer for the photovoltaic layer are applications making effective use of the feature that the light sensitivity thereof is very close to the sensitivity of the human eye. Also, it is superior for detecting long wavelength light. However, it has been found that the a-SiGe photovoltaic layer of the photovoltaic element of the present invention is different from conventional a-SiGe in terms of Raman scattering, electron beam diffraction, and high resolution electron microscopic observation
Conventionally, microcrystalline germanium (mc-Ge) has been used for temperature sensors, optical power sensors, and infrared sensors. Examples are shown in "A high-accuracy quick-response optical power sensor with .mu.c-Ge:H thin film", S. Kodato, Y. Naito, K. Kuroda, S. Kodama, Sen Actuators A, vol. 28, No. 1, pp. 63-68, and "Thermoelectric characteristics of microcrystalline germanium thin film and its application to sensors", Etsunobu Naito, Yasuji Uchida, Kazuo Mizuno, Setsuo Yoshidatsuchi, Institute of Electrical Engineers of Japan, Sensor Technology Research Meeting on Materials.
However, a photovoltaic element with mc-Ge contained in the photovoltaic layer has not been reported. Also, there have been few researches concerning such application, resulting in too low a quality for application to photovoltaic elements.
On the other hand, regarding the film forming apparatus, several studies for making photovoltaic elements have been conducted using the MWPCVD method, which has a higher deposition rate and is superior in source gas utilization efficiency. For example, the following article discloses forming the i-layer by a MWPCVD method: "a-Si solar cell by microwave plasma CVD", Kazufumi Higashi, Takeshi Watanabe, Toshikazu Shimada, Proceedings of The 50th Scientific Lecture Meeting in The Japanese Society of Applied Physics, p. 566. In this photovoltaic element, a good quality i-layer is obtained at higher deposition rate by MWPCVD.
Also, examples of articles disclosing forming the doping layer by MWPCVD include:
"High Efficiency Amorphous Solar Cell Employing ECR-CVD Produced p-type Microcrystalline SiC Film", Y. Hattori, D. Kruangam, T. Toyama, H. Okamoto and Y. Hamakawa, Proceedings of the International PVSEC-3, Tokyo, Japan (1987) p. 171, and
"HIGH-CONDUCTIVE WIDE BAND GAP P-TYPE a-SiC:H PREPARED BY ECR CVD AND ITS APPLICATION TO HIGH EFFICIENCY a-Si BASIS SOLAR CELLS", Y Hattori D. Kruangam, K. Katou, Y. Nitta, H. Okamoto and Y. Hamakawa, Proceedings of 19th IEEE Photovoltaic Specialists Conference (1987) p. 689.
In these photovoltaic elements, a good-quality p-layer is obtained by using the MWPCVD method for forming the p-layer. However, in these examples, mc-Ge is not referred to.
The above-mentioned a-SiGe solar cell had a problem that the open-circuit voltage is too low and the fill factor (FF) is small, although the a-SiGe layer can be used in the photovoltaic layer to efficiently convert light ranging from short wave to long wave into electric power. Further, if the a-SiGe layer is used in the photovoltaic layer, light deterioration (reduced electrical characteristics after subjecting the photoelectric conversion element to light illumination for a long time) is problematical. Also, photosensors using the a-SiGe layer as the photoelectric conversion layer had a problem with the residual image characteristic.